The present invention relates to a technique for supplying voltage to a starting circuit which generates a high voltage starting signal for an electric discharge lamp, provided in a lighting circuit of an electric discharge lamp.
Concerning a lighting circuit for lighting an electric discharge lamp (such as a metal halide lamp), a structure is well known which includes a DC electric power source circuit, DC-AC conversion circuit and starting circuit (what is called a starter circuit). For example, a DC-DC converter is used in the DC electric power source circuit, and a full bridge type circuit (four semiconductor switch elements (or switching elements) are formed into two pairs so as to conduct switching control) and its driver circuit are used in the DC-AC conversion circuit. In the above structure, the voltage of positive or negative polarity output by the DC-DC converter is converted into a rectangular wave voltage by the full bridge type circuit and supplied to an electric discharge lamp.
In this connection, concerning the starting circuit, for example, the starting circuit includes a transformer, a condenser provided on the primary winding side and a self-yielding type switch element. When the voltage across both terminals of the condenser exceeds a threshold value, the self-yielding type switch element yields to the voltage, so that a primary current flows in the circuit and the voltage of this current is boosted and impressed upon the electric discharge lamp from the secondary winding.
In this connection, it is well known that in the case of no load in which an electric discharge lamp has not been turned on yet, the higher the voltage is {what is called an open circuit voltage or open voltage, which is temporarily output by the DC electric power source circuit (DC-AC converter)), the more easily the electric discharge lamp can be turned on.
However, when the above open circuit voltage is raised too high, it becomes necessary to increase the withstanding voltages of the DC electric power source circuit and the DC-AC conversion circuit, which causes an increase in the manufacturing cost. Therefore, the upper limit of above open circuit voltage is restricted.
In the case of generating the starting voltage for starting the electric discharge lamp, if it is possible to generate a starting pulse (or starter pulse) by supplying an open circuit voltage, which is obtained from the DC electric power supply circuit or DC-AC conversion circuit, to the primary circuit of the starting circuit, the number of parts composing the circuit can be reduced to be minimum. However, in order to obtain a pulse, the wave length of which is sufficiently high for starting the electric discharge lamp, it is necessary to increase a boosting ratio of the transformer provided in the starting circuit. In order to increase the boosting ratio of the transformer, it is necessary to increase the inductance of the secondary winding of the transformer concerned. As a result, the size of the transformer is increased, which raises the manufacturing cost.
It is possible to use a structure in which the output voltages of the DC electric power source circuit and the DC-AC conversion circuit are boosted up by a voltage doubler circuit, which is newly provided, including a condenser and diode, and then the thus boosted voltage is supplied to the starting circuit. It is also possible to use a structure in which a new secondary winding is provided on the secondary side of the converter transformer composing the DC electric power source circuit, and a high voltage output is supplied from the winding concerned to the starting circuit.
However, according to the above circuit structure, it is necessary to increase a withstanding voltage of the line through which the output voltage of the voltage doubler circuit is impressed. It is also necessary to increase a withstanding voltage of the line through which the output voltage of the secondary winding is supplied to the starting circuit. Therefore, severe design constraints are imposed on the circuit so that it can withstand the high voltage.
When the voltage supplied to the starting circuit (the primary circuit of the starting circuit) is raised, safety problems may be encountered concerning the wiring. That is, when the starting circuit is arranged close to the electric discharge lamp (for example, when the lighting device is attached to the starting circuit), it is necessary to lay two electric power supply lines from the output terminals of the lighting circuit to the electric discharge lamp, and further it is necessary to lay one voltage supply line from the output terminal of the lighting circuit to the starting circuit. From the viewpoint of preventing accidental electrocution, it is preferable that the voltage of the voltage supply line is set as low as possible, that is, it is preferable that the electrical potential of the voltage supply line with respect to the ground electric potential (GND) is low.
The present invention reduces a withstanding voltage of a voltage supply line to a starting circuit of a lighting circuit to be as low as possible so as to enhance safety by reducing the difference in electric potential of the voltage supply line with respect to the ground electric potential. In addition, the present invention reduces the manufacturing cost.
Presented is a lighting circuit of an electric discharge lamp that includes a DC electric power source circuit for outputting a DC voltage; a DC-AC conversion circuit for converting an output voltage of the DC electric power source circuit into an AC voltage so as to supply the AC voltage to an electric discharge lamp; and a starting circuit for generating a high voltage starting signal to start the electric discharge lamp. The lighting circuit of an electric discharge lamp has the structure shown in the following items (a) to (d).
(a) The starting circuit includes a transformer including a magnetic body, primary winding and secondary winding, and a primary circuit provided on the primary side of the transformer.
(b) The primary circuit includes the primary winding and a series circuit of a condenser and a switch element, and the series circuit is connected in parallel with the primary winding.
(c) When or after the voltage across both terminals of the condenser exceeds a threshold value in accordance with the accumulation of electrical charge in the condenser, the switch element is electrically continued so that the high voltage starting signal is impressed upon the electrical discharge lamp from the primary winding via the secondary winding.
(d) When the output voltage of the DC electric power source circuit is positive with respect to the ground electric potential, the polarity of the supply voltage supplied to the primary circuit for charging the condenser in the primary circuit according to the output from the DC electric power source circuit or DC-AC conversion circuit is made to be a negative polarity, and when the output voltage of the DC electric power source circuit is negative with respect to the ground electric potential, the polarity of the supply voltage supplied to the primary circuit for charging the condenser in the primary circuit according to the output from the DC electric power source circuit or DC-AC conversion circuit is made to be a positive polarity.
Consequently, according to the present invention, when the supply voltage, the polarity of which is the reverse of the polarity of the output voltage of the DC electric power source circuit, is supplied to the primary circuit of the starting circuit, the withstanding voltage of the voltage supply line to the circuit concerned can be reduced.